1. Field of the Invention
The present invention relates to an optical modulator controlling the phase of an optical signal, and the like. In particular, the present invention relates to an optical modulator and the like which is used for FSK-PSK modulation by controlling the phase of an optical FSK modulation signal.
2. Description of the Related Art
In optical communication, light must be modulated to have signals. As optical modulations, direct modulation and external modulation are known. The direct modulation modulates a driving power of semiconductor laser. And the external modulation modulates light from semiconductor laser by means other than a light source. A modulator used in external modulation is generally referred to an optical modulator. The optical modulator modulates optical intensity, phase, etc. by causing physical changes in the optical modulator based on signals. As technical problems of the optical modulator, there exist reduction of driving voltage, realization of a higher extinction ratio for improving modulation efficiency, widening a bandwidth, and improvement of high light utilization efficiency for speeding up and loss reduction of a modulation. In other words, development of a modulator having high extinction ratio is desired. It is to be noted that the extinction ratio is a ratio of optical intensity between the highest level and the lowest level.
As an example which shifts a frequency of an optical signal to be outputted, there is an optical signal side-band (optical SSB) modulator [Tetsuya Kawanishi and Masayuki Izutsu, “Optical frequency shifter using optical SSB modulator”, TECHNICAL REPORT OF IEICE, OCS2002-49, PS2002-33, OFT2002-30 (2002-08)].
An optical FSK modulator which is a modification of an optical SSB modulator is also known [Tetsuya Kawanishi and Masayuki Izutsu, “Optical FSK modulator using an integrated light wave circuit consisting of four optical phase modulator”, CPT 2004G-2, Tokyo, Japan, 14-16 Jan. 2004] [Tetsuya Kawanishi, et al. “Analysis and application of FSK/IM simultaneous modulation” Tech. Rep. of IEICE. EMD 2004-47, CPM 2004-73, OPE 2004-130, LQE 2004-45 (2004-08), pp. 41-46].
In the optical FSK modulator, when a modulation signal applied to a sub Mach-Zehnder waveguide is represented by fm [Hz], an upper side band (USB) signal (f0+fm) or a lower side band (LSB) signal (f0+fm) is used as frequency shift information. This is why the phases of the USB signal and the LSB signal are not considered, and a phase difference between the USB signal and the LSB signal exists. But in an FSK-PSK modulation, since phase information is added to an optical FSK signal, the phase difference between the USB signal and the LSB signal is a problem. If the phase difference between the USB signal and the LSB signal of the FSK modulator is able to be controlled, various uses thereof are expected.
This invention is made to provide an optical modulator capable of controlling the optical phase of various kinds of output signals to be, for example, 0 degree or 180 degrees. This invention, by controlling the phase difference of an USB signal and an LSB signal of an optical FSK modulator, is also made to provides embodiments of optical FSK modulation system which have not been considered in the prior art.
This invention is also made to provide an optical FSK modulator capable of controlling the phase of an USB signal and the phase of an LSB signal of an optical FSK modulation signal.
This invention is also made to provide an FSK-PSK modulation system.